Generally, uplink channels for a currently discussed communication system include a random access channel (RACH or a ranging channel) for a user equipment to randomly access a base station, an uplink shared channel (e.g., HS-DPCCH) for carrying a channel quality indicator (CQI) and ACK/NACK information, and the like.
The RACH or ranging channel is a random access channel for a user equipment to perform downlink synchronization with a base station and can be found through bases station information. A location of a corresponding channel and the like can be acquired from the base station information. And, the RACH or ranging channel is a unique channel that can be accessed by a user terminal which is not synchronized with a base station yet.
If a user equipment transmits a signal to a corresponding base station on the RACH or ranging channel, the base station informs the user equipment of modification information on an uplink signal timing for synchronization with the base station and various information for the corresponding user equipment to be connected to the base station.
After a connection between the user equipment and the base station has been completed through the RACH or the ranging channel, communications can be carried out using other channels.
FIG. 1 and FIG. 2 are diagrams for examples of a process generated when a user equipment connects an uplink communication with a base station.
First of all, a user equipment can acquire both uplink and downlink synchronizations with a base station by accessing an RACH or a ranging channel. So, the user equipment is in a state capable of accessing the corresponding base station. FIG. 1 shows a situation that a user equipment is initially connected to a base station after a power of the user equipment has been turned on. FIG. 2 shows that a user equipment having performed initial synchronization with a base station accesses the base station if the synchronization is mismatched or if a request for an uplink resource needs to be made (i.e., if a resource for uplink transmission data is requested).
In a step (1) of FIG. 1 or FIG. 2, a user equipment transmits an access preamble and a message to a base station if necessary. The base station recognizes why the corresponding user equipment accesses an RACH or a ranging channel and then makes an action for a corresponding process.
In case of the initial access shown in FIG. 1 or FIG. 2, the base station allocates timing information and an uplink data resource to the corresponding user equipment in steps (2) and (3). So, the user equipment is able to transmit uplink data in a step (4).
FIG. 2 shows an example of a case that the user equipment accesses the RACH or the ranging channel in the step (1) because of a scheduling request (hereinafter abbreviated SR). In the step (2), the base station performs resource allocation for the timing information and the SR to the user equipment. For the SR (step (3) of the user equipment, the base station performs uplink data resource allocation [step (4)] to enable the user equipment to perform uplink data transmission [step (5)]. In case that the SR is transmitted on a random access channel, it means a case that the user equipment having been in an idle/sleep mode for a long time is decided to have a timing mismatched with that of the base station. So, this scheme enables both of the timing information and the resource allocation request to be handled at a time.
In accessing the RACH or the ranging channel, in case of the case shown in FIG. 2 instead of the initial access, a different signal is usable according to whether a signal carried on the RACH or the ranging channel is matched in synchronization with the base station.
FIG. 3 is a diagram for a structure of an RACH or a ranging channel used for a synchronous/asynchronous access.
In case of a synchronized access, a user equipment having performed synchronization with a base station makes an access to a RACH or a ranging channel in a situation that the synchronization is maintained (synchronization can be maintained through control information such as a downlink signal or a CQ pilot transmitted in uplink). And, the base station is facilitated to recognize a signal carried on the RACH or the ranging channel.
Since the synchronization is being maintained, the user equipment, as shown in an upper part of FIG. 3, is able to use a longer sequence or further transmit additional data.
In case of a non-synchronized access, when a user equipment makes an access to a base station, if synchronization is mismatched due to some cause, a guard time, as shown in a lower part of FIG. 3, should be set in accessing an RACH or a ranging channel. The guard time is set by considering a maximum roundtrip delay that a user equipment attempting to receive a service within the base station can have.
The RACH or the ranging channel should vary in length according to a cell size of the base station. As the user equipment gets farther from the base station, a round-trip delay gets increased. And, this means that the guard time set for the user equipment in the non-synchronized access gets longer. If the cell size is increased, a path loss between the user equipment and the base station is increased. So, a signal needs to be transmitted by being spread longer, which is shown in FIG. 4.
FIG. 4 is a diagram to explain a cell size and a channel length.
Referring to FIG. 4, a length of a channel, and more particularly, a length of an RACH or a ranging channel is set proportional to a cell size at a place where a communication will be actually installed. FIG. 4 shows three kinds of RACHs according to a rule of categorizing cell sizes into a small cell size, a medium cell size, and a large cell size. And, a different sequence is applied to each of the RACHs respectively having three kinds of lengths, which is indicated by a different shaded portion. In particular, how an inside of a cell is segmented can be diversified according to a condition of a corresponding system. And, a scheme for extending the length of the RACH or the ranging channel and a sequence applied thereto can be diverse as well.
Meanwhile, a user equipment transmits a signal via an RACH or a ranging channel. This is because the user equipment can obtain a specific service in a manner of transmitting a selected sequence to a base station to match a synchronization of an uplink signal to the corresponding base station. To achieve this object, entire user equipments within an area defined as a cell should have success probability over a predetermined level regardless of a location of the corresponding user equipment. For this, in case that a cell size is small, a variation of an RACH or ranging channel resource is not considerable. So, a quantity occupied by an RACH or a ranging channel in an overall system is very small. For instance, in case that 1 subframe is used as an RACH or a ranging channel in 3GPP LTE system, the system uses 1/20 of overhead as the RACH or the ranging channel. Yet, if 5 subframes need to be used due to an increased cell size, the overhead increases 5 times to considerably affect overall system performance.
As a scheme for reducing the overhead in a large cell, a method of changing a cycle of an RACH or a ranging channel can be taken into consideration. Yet, this method raises a problem that an access latency is elongated when a user equipment access the RACH or the ranging channel. And, it is also disadvantageous that probability of collision occurrence in an RACH or ranging channel slot is raised.
In case that entire user equipments within a large cell use an identically specified sequence, probability of collision in an RACH or ranging channel slot can be raised in proportion to an increasing number of user equipments within the corresponding cell.
Accordingly, the demand for a technology in reducing probability of collision occurrence in the same RACH or ranging channel slot and overhead attributed to an RACH or a ranging channel in a large cell has risen.
However, a detailed scheme for solving the problem has not been proposed.